The invention concerns in particular, but not exclusively, electronic devices wherein one or more transistors and one or more passive components are integrated monolithically, and provides for the formation of a magnetic circuit structure concurrently therewith. The description to follow makes reference to this application of the invention for convenience of illustration only.
Passive elements, such as resistors and capacitors, are easily formed along with active components (diodes and transistors) in semiconductor electronic devices. For certain applications, the availability of inductors would be highly desirable. Circuit configurations which haven't been integrated so far because of the unavailability of such components, would then be possible.
Unfortunately, one of the features inherent to a magnetic circuit is the need for large areas or volumes in order to produce acceptable inductance values. The main difficulty lies in the formation of a structure which is three-dimensional in concept within an integrated circuit which is bi-dimensional in nature (all the active structures are located within a few microns from one of the two surfaces).
As is well known, to form a magnetic circuit in a semiconductor component one must integrate on the silicon one or more inductors generating a magnetic field. An adequately exhaustive survey of practicable implementations is contained in U.S. Pat. No. 5,095,357. FIGS. 1, 2, 3, 6, 9, 10, 11, 12, 13, 14 of that document illustrate different solutions to the commonest problems posed by that, i.e. the magnetic circuit is a three-dimensional element in concept, the host integrated circuit is basically a bi-dimensional structure.
A feature shared by all of the solutions proposed in the above-referenced document, but for that shown in FIG. 13, is the generation of a magnetic field perpendicular to the semiconductor substrate. An evident disadvantage of this feature is represented by the induction of the magnetic field in the substrate altering the performance of the active components therein.
It's known from the fundamental laws of physics and electromagnetism that any magnetic circuit produces magnetic induction and an associated electric field. Also known to anyone of ordinary skill in the art of integrated circuits is that the operation of an active component, such as an integrated transistor, is based on the movement of charges of opposite signs through a region of the semiconductor material. Thus, it will be apparent to those skilled in the art that the presence of magnetic fields with lines of force perpendicular and/or parallel to the silicon surface, unless suitably controlled, may cause considerable disturbance and make the operation of the electronic components in the integrated circuit unpredictable. An example of this phenomenon, albeit referred to a magnetic field perpendicular to the substrate, is illustrated in FIG. 2. Another disadvantage of the solutions listed in the reference, and illustrated by FIGS. 1 to 12, comes from the area required to form the coils and from the complexity inherent to providing a number of coupled coils, when transformers are to be formed.
A further disadvantage of such solutions is the capacitive coupling to the substrate of the conductors of which the coils are made. FIG. 13 shows a magnetic component wherein the flux lies parallel to the surface of the semiconductor substrate.
While achieving its objective, not even this solution is entirely devoid of shortcomings. The circuit of FIG. 13, although parallel to the surface of the semiconductor substrate, also generates a magnetic field whose flux lines fan out into the environment and, therefore, into the semiconductor material, at both ends of the winding.
In addition, the inductor winding forming part of the magnetic circuit extends over both faces of the substrate, that is, each element of the coil is passed successively from one face to the other of the substrate by connections which extend through the substrate. It will be appreciated that this is an attempt at bringing the magnetic circuit back to a three-dimensional configuration, while the world of integrated circuits is basically bi-dimensional.
Those skilled in the art will recognize that this technique is very unusual in the manufacturing of integrated circuits, and its adoption would entail substantial modifications to the standard manufacturing methods, including the application of lithographic processes to both surfaces of the semiconductor substrate.
Furthermore, the problem of the magnetic field induced in the substrate remains unsolved, even though this problem is somewhat mitigated by the field lines being parallel to the substrate surface, whereby some of the flux is passed into the air and some into the substrate. Easily seen from the figures are, on the other hand, the complications involved in the manufacturing of structures with multiple coupled windings (FIGS. 13a-e).
Yet another disadvantage associated with the aforementioned prior art is that all the solutions proposed have an open magnetic flux: FIGS. 12 to 15 showing solutions which are of an extreme complexity and less than fully successful in their attempt at producing transformers against such a limitation.
This invention concerns structures of a magnetic device which can be integrated on a semiconductor substrate, consistent with the manufacturing techniques for a standard integrated circuit. These structures provide improved performance over the prior art (including reduced area consumption), prevent magnetic flux losses (that is, ensure better utilization of the magnetic flux generated), and reduce the capacitive coupling to the substrate, while allowing the integration of transformers on a semiconductor substrate concurrently with an integrated circuit. The transformers occupy a limited area and have high magnetic coupling capabilities, thereby overcoming the aforementioned limitations and/or drawbacks of the prior art.